Transistors find use in a wide range of integrated circuits. One type of transistor is the metal-oxide-semiconductor field effect transistor or MOSFET. Typical MOSFETs include source/drain diffusion regions which are disposed within a substrate and a conductive gate which overlies a channel region intermediate the source/drain diffusion regions. In some MOSFETs, placement of a desired voltage on the conductive gate enables a channel to be formed between the source/drain diffusion regions. With the channel being formed, current can be made to flow between the source and the drain. There are a number of different types of MOSFETs such as NMOS and PMOS field effect transistors.
NMOS field effect transistors are typically formed on a p-type substrate or p-well. The channel in an NMOS transistor is usually formed through provision of a positive gate voltage on the transistor which attracts minority electrons within the p-type substrate into the channel region. PMOS field effect transistors are typically formed on an n-type substrate or n-well. The channel in PMOS transistors is typically formed through provision of a negative gate voltage on the transistor gate which attracts minority holes from the n-type substrate into the channel region to form the channel. CMOS (Complementary Metal Oxide Semiconductor) transistors utilize both NMOS and PMOS transistors.
An important parameter in MOS transistors in the threshold voltage V.sub.t, which is the minimum gate voltage required to induce the channel. In general, the positive gate voltage of an n-channel device (NMOS) must be larger than some value V.sub.t before a conducting channel is induced. Similarly, a p-channel device (PMOS) requires a gate voltage which is more negative than some threshold value to induce the required positive charge in the channel. A valuable tool for controlling threshold voltage is ion implantation. Because very precise quantities of impurity can be introduced into the substrate by this method, it is possible to maintain close control of V.sub.t. For example, introduction of a p-type impurity into a p-channel PMOS device can make V.sub.t less negative. This is because the channel region is made more p-type and therefor a lower magnitude of negative voltage is required to induce holes within the channel region.
As MOS transistors are made smaller and smaller, they become susceptible to so-called short channel effects. Short channel effects can be divided into (a) those that impact V.sub.t, (b) those than impact subthreshold currents, and (c) those that impact I-V behavior beyond threshold. Short channel effects include punch through which normally occurs as a result of the widening of the drain depletion region when the reverse-bias voltage on the drain is increased. The electric field of the drain may eventually penetrate into the source region and thereby reduce the potential energy barrier of the source-to-body junction. When this occurs, more majority carriers in the source region have enough energy to overcome the barrier, and an increased current then flows from the source to the body, some of which is collected by the drain. One way of reducing the electric field of the drain and hence the risk of punch through is to form so-called halo regions proximate the source/drain regions of a transistor. Halo regions are described in more detail in the texts which are incorporated by reference below.
One type of MOS transistor is a buried channel PMOS. Buried channel PMOS transistors typically have a p- diffusion region disposed within the substrate underneath the gate and between the source/drain diffusion regions. The elevational thickness of the p- diffusion region within the substrate is referred to as gamma-j. One goal in the design of buried channel PMOS transistors is to reduce gamma-j to improve control over current leakage. It is also desirable to provide the p-region as close to the gate as possible to provide for more gate control.
For a more detailed treatment of short channel effects and other relevant semiconductor processing concerns, the reader is referred to two texts: Wolf, Silicon Processing for the VLSI Era, Volume 2, Chapters 5 and 6; and Streetman, Solid State Electronic Devices, Fourth Edition, both of which are expressly incorporated by reference herein.
This invention arose out of concerns associated with improving the methodology through which MOS devices are fabricated, and improving the resultant MOS structures.